The present invention pertains generally to the field of sensors for the measurement of changes in mass and heat flow. More particularly, the present invention pertains to a combined microresonator mass sensor and heat flow sensor which may provide simultaneous and continuous measurement of the changes in mass and heat flow at a gas-solid interface.
Although the piezoelectric effect has been known since the 19th century, the development of quartz crystal devices which oscillate at precisely defined resonant frequencies and which can be incorporated as passive elements into electronic instruments began in the 1920""s. Like much of our modern electronic technology, their development received a massive push during World War II, when over 30 million quartz crystal oscillators were produced for use in military communications equipment. Today there is widespread use of quartz crystal oscillators and of newer types of microresonators in electronics wherever precise control of frequency is needed as, for example, in radio frequency communications, in frequency meters and timepieces, in scientific instrumentation, and in computers and cellular telephones.
There are several useful books which describe the physics of quartz crystal oscillators and other microresonators and their use in electronic circuits. For example, Introduction to Quartz Crystal Unit Design by Bottom, Van Nostrand Reinhold, New York, 1982, discusses the physical crystallography of quartz, mechanic vibrations and stress/strain relationships, the piezoelectric effect, the equivalent circuit of the quartz resonator and its use as a circuit component, the temperature stability of quartz oscillators, and other topics of importance in the application of these devices. Science, Vol. 249, pages 1000-1007 (1990), by Ward et al., describes the converse piezoelectric effect and its use in in-situ interfacial mass detection, such as in thickness monitors for thin-film preparation and in chemical sensors for trace gases. Analytical Chemistry, Vol. 65, pages 940A-948A and 987A-996A (1993), by Grate et al., compares the acoustical and electrical properties of five acoustic wave devices used as microsensors and transducers, including quartz crystal oscillators.
Any crystalline solid can undergo mechanical vibrations with minimum energy input at a series of resonant frequencies, determined by the shape and size of the crystal and by its elastic constants. In quartz, such vibrations can be induced by the application of a radio frequency voltage at the mechanical resonant frequency across electrodes attached to the crystal. This is termed the inverse piezoelectric effect. The thickness shear mode is the most common mechanical vibration used in quartz crystal oscillators. A typical commercially available quartz crystal oscillator is a thin circular quartz plate, cut from a single crystal at an angle of 37.25xc2x0 with respect to the crystal""s z axis (the so-called xe2x80x9cAT cutxe2x80x9d). This angle is chosen so that the temperature coefficient of the change in frequency is, to the first approximation, zero at 25xc2x0 C., thus minimizing the drift in resonant frequency with ambient temperature change. A slight change in the cut angle produces crystals with zero temperature coefficients at elevated temperatures. The AT-cut plate has thin film electrodes on most of the top and bottom surfaces of the crystal, and is supported in various ways at its circumference or perimeter. Both the fundamental and the first few overtones of the thickness shear mode have been utilized in crystal oscillators. A typical AT-cut quartz disk piezoid operating at a 10.8 MHz fundamental has the following dimensions, according to page 99 of the above-mentioned reference by Bottom:
The quality factor, Q, defined for any resonant circuit incorporating quartz crystal oscillators is usually not less than 105 and may be as high as 107. With careful attention to the control of temperature in a vacuum environment, a short-term frequency stability of one part in 1010 can be obtained, although the stated short-term stability for commercial units is xc2x13 ppm.
The resonant frequency of a quartz crystal oscillator is inversely proportional to the thickness, e, of the plate. For a circular disk,
f=nK/e 
where n=1, 3, 5, . . . and K is the frequency constant (for example, see page 134 ff of the above-mentioned reference by Bottom). For an AT-cut disk, K=1664 kHzxe2x80xa2mm, so that a disk of a thickness of 1 mm will oscillate at 1.664 MHz. If this thickness is increased by the deposition of material on the surface of the quartz crystal oscillator, then its frequency will decrease.
In 1957, Sauerbrey in Z. Physik, Vol. 155, 206 (1959), derived the fractional decrease in frequency xcex94f of a circular disk quartz crystal oscillator upon deposition of a mass, xcex94m, of material on its surface. The derivation relies on the assumption that a deposited foreign material exists entirely at the anti-node of the standing wave propagating across the thickness of the quartz crystal, so that the foreign deposit can be treated as an extension of the crystal, as, for example, described in Applications of Piezoelectric Quartz Crystal Microbalances by Lu et al., Elsevier, New York, 1984. Sauerbrey""s result for the fundamental vibrational mode is as follows:       Δ    ⁢          xe2x80x83        ⁢          f      /              f        0              =                    -        Δ            ⁢              xe2x80x83            ⁢              e        /                  e          0                      =                  -        2            ⁢              f        0            ⁢      Δ      ⁢              xe2x80x83            ⁢              m        /        A            ⁢              ρ            ⁢      μ      
Here, xcex94e is the change in the original thickness e0, A is the piezoelectrically active area, xcfx81 is the density of quartz, and xcexc is the shear modulus of quartz. By measuring the decrease in frequency, one thus can determine the mass of material deposited on the crystal. This is the principle of the quartz crystal microbalance. In practice, the assumptions underlying the Sauerbrey equation are valid for deposits up to 10% of the crystal mass, although the sensitivity to mass has been shown experimentally to decrease from the center of the electrode to its edge.
Torres et al. in J. Chem. Ed., Vol. 72, pages 67-70 (1995), describe the use of a quartz crystal microbalance to measure the mass effusing from Knudsen effusion cells at varying temperatures, in order to determine the enthalpies of sublimation. They reported a sensitivity of about 10xe2x88x928 g/sec in the mass deposition rate. The application of the quartz crystal microbalance and other microresonators in chemistry for the sensitive detection of gases adsorbed on solid absorbing surfaces has been reviewed by Alder et al., in Analyst, Vol. 108, pages 1169-1189 (1983) and by McCallum in Analyst, Vol. 114, pages 1173-1189 (1989). The quartz crystal microbalance principle has been applied to the development of thickness monitors in the production of thin films by vacuum evaporation, as, for example, described in the above-mentioned reference by Lu et al. Quartz crystal oscillators of various sizes and modes of vibration are commonly used currently in research efforts in sensor development.
Throughout this application, various publications and patents are referred to by an identifying citation. The disclosures of the publications and patents referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
U.S. Pat. No. 5,339,051 to Koehler et al. describes resonator-oscillators for use as sensors in a variety of applications. U.S. Pat. No. 4,596,697 to Ballato and U.S. Pat. No. 5,151,110 to Bein et al. describe coated resonators for use as chemical sensors.
To overcome the influences of temperature changes on the microresonators, U.S. Pat. No. 4,561,286 to Sekler et al. and U.S. Pat. No. 5,476,002 to Bower et al. describe active temperature control or the use of temperature sensors with the microresonators. U.S. Pat. No. 5,686,779 to Vig describes a microresonator for direct use as a thermal sensor.
Microresonators, including quartz crystal microbalances (QCM""s), have been utilized to determine the mass changes with a variety of liquid samples such as, for example, described in U.S. Pat. No. 4,788,466 to Paul et al. When the microresonator is coated, chemicals present in the liquid samples may be detected as, for example, described in U.S. Pat. No. 5,306,644 to Myerholtz et al.
Microresonators have been adapted to measure the viscosity of a liquid sample as, for example, described in U.S. Pat. No. 4,741,200 to Hammerle. U.S. Pat. No. 5,201,215 to Granstaff et al. describes the use of microresonators to measure the mass of a solid and physical properties of a fluid in a sample.
Calorimeters for various types of heat measurements are well known as, for example, described in U.S. Pat. No. 4,492,480 to Wadso et al.; U.S. Pat. No. 5,295,745 to Cassettari et al.; and U.S. Pat. No. 5,312,587 to Templer et al. A combined scientific apparatus of a thermal analyzer, such as a calorimeter, and an X-ray diffractometer for observing simultaneously both thermodynamic and structural properties of materials is described in U.S. Pat. No. 4,821,303 to Fawcett et al.
Despite the various approaches proposed for the design of sensors based on microresonators as the sampling device, there remains a need for sensors which can simultaneously and continuously measure with high sensitivity and accuracy both mass and heat flow changes of a sample in contact with the microresonator.
One aspect of the present invention pertains to a new scientific instrument or device based on the combination of: (i) a microresonator, such as, for example, a quartz crystal microbalance (QCM), which may be used to measure very small changes of mass at its surface; and, (ii) a heat flow sensor, such as, for example, an isothermal heat conduction calorimeter (HCC), which may be used to measure small heat flows. In one embodiment, the microresonator and heat flow sensor combination measures simultaneously and continuously, with high sensitivity (nanogram in mass, sub-microwatt in heat flow), the changes in mass and heat flow at a small gas-solid interface, for example, about 1 cm2 or less in area, due to a chemical process such as evaporation or condensation, adsorption or desorption, or gas-surface reactions. The new scientific device of the present invention may be advantageously utilized in a variety of applications such as, for example, studying the hydration and dehydration of films of proteins and other biomolecules deposited on solid substrates, particularly for films utilized in biosensors, diagnostic immunoassays, the separation of proteins by chromatography, and as models for biological and biocompatible membranes and surfaces; studying the energetics of intermolecular interactions at the surface of polymer films and other organic surfaces important in adhesion, lubrication, wetting, and corrosion; and studying the energetics of the drying and curing of both water-based and organic solvent-based paints and finishes.
One aspect of the present invention pertains to a mass and heat flow measurement sensor comprising (i) a microresonator comprising a piezoelectric substrate having a perimeter, a first face for directly contacting a sample, and a second opposite face isolated from contacting the sample, the piezoelectric substrate having a resonant frequency and capable of producing a measurement signal based on the resonant frequency; (ii) a heat flow sensor coupled thermally to the piezoelectric substrate of the microresonator; and, (iii) a heat sink coupled thermally to the heat flow sensor. In one embodiment, the microresonator has the capability to measure the mass of the sample applied to the first face, and the heat flow sensor has the capability to measure the flow of heat from the sample on the first face of the microresonator to the heat sink.
Suitable microresonators for the sensors of this invention include, but are not limited to, bulk acoustic wave sensors, quartz crystal microbalances, surface acoustic wave sensors, flexural plate wave sensors, and acoustic plate mode sensors. In a preferred embodiment, the microresonator is a quartz crystal microbalance.
Another aspect of the present invention pertains to a mass and heat flow measurement sensor comprising (i) a microresonator comprising a piezoelectric substrate having a perimeter, a first face for directly contacting a sample, and a second opposite face isolated from contacting the sample; (ii) electrodes deposited on the first and second opposite faces of the piezoelectric substrate, the electrodes being capable of supplying electrical signals to and from the piezoelectric substrate; (iii) a heat flow sensor; (iv) a heat conductive material extending in a continuous fashion from the perimeter of the piezoelectric substrate to a first surface of the heat flow sensor, wherein the heat conductive material is not in contact with an acoustically active region of the second opposite face; and, (v) a heat sink material in contact to a second surface of the heat flow sensor, which second surface is not in direct contact with the heat conductive material. In one embodiment, the microresonator has the capability to measure the mass of the sample applied to the first face, and the heat flow sensor has the capability to measure the flow of heat from the sample on the first face of the microresonator to the heat sink. In a preferred embodiment, the heat flow sensor comprises a thermopile.
In one embodiment of the sensors of this invention, the piezoelectric substrate is a heat conductive material. In one embodiment, the piezoelectric substrate is a quartz crystal, and, preferably, the piezoelectric substrate is an AT-cut quartz crystal.
In one embodiment of the sensors of the present invention, the heat conductive material provides a path for the application of radio-frequency power to the piezoelectric substrate. In one embodiment, the heat conductive material comprises a metallic cylinder with a surface in contact to the first surface of the heat flow sensor. In one embodiment, the heat conductive material is brass.
One aspect of the present invention pertains to a mass and heat flow measurement sensor comprising (i) a quartz crystal microbalance capable of measuring the mass of a sample in contact with the quartz crystal microbalance; (ii) a heat flow sensor coupled thermally to the quartz crystal microbalance and capable of measuring the flow of heat from the sample to a heat sink; and, (iii) a heat sink coupled thermally to the heat flow sensor. In one embodiment, the quartz crystal microbalance comprises a quartz substrate having a perimeter, a first face for directly contacting the sample, and a second opposite face isolated from contacting the sample, the quartz substrate having a resonant frequency and capable of producing a measurement signal based on the resonant frequency. In one embodiment, the mass and heat flow measurement sensor further comprises electrodes deposited on the first and second opposite faces of the quartz substrate, the electrodes being capable of supplying electrical signals to and from the quartz substrate. In one embodiment, a heat conductive material extends in a continuous fashion from the perimeter of the quartz substrate to a first surface of the heat flow sensor, wherein the heat conductive material is not in contact with an acoustically active region of the second opposite face. In a preferred embodiment, the heat flow sensor comprises a thermopile. In one embodiment, the heat sink material is in contact to a second surface of the heat flow sensor, which second surface is not in direct contact with the heat conductive material. In a preferred embodiment, the quartz substrate is an AT-cut quartz crystal. In one embodiment, the heat conductive material provides a path for the application of radio-frequency power to the quartz substrate. In one embodiment, the heat conductive material comprises a metallic cylinder with a surface in contact to the first surface of the heat flow sensor. In one embodiment, the heat conductive material is brass.
Another aspect of the present invention relates to methods for measuring the mass of a sample and the flow of heat from the sample to a heat sink, which methods comprise the steps of: (i) providing a microresonator, as described herein; (ii) providing a heat flow sensor coupled thermally to the piezoelectric substrate of the microresonator; (iii) providing a heat sink coupled thermally to the heat flow sensor; and, (iv) measuring the changes in mass of the sample and the flow of heat from the sample to the heat sink for the sample disposed on the first face of the piezoelectric substrate.
Still another aspect of the present invention relates to methods for measuring the mass of a sample and the flow of heat from the sample to a heat sink, which methods comprise the steps of: (i) providing a microresonator, as described herein, (ii) providing electrodes deposited on the first and second opposite faces of the piezoelectric substrate, the electrodes being capable of supplying electrical signals to and from the piezoelectric substrate; (iii) providing a heat flow sensor, as described herein; (iv) providing a heat conductive material extending in a continuous fashion from the perimeter of the piezoelectric substrate to a first surface of the heat flow sensor, wherein the heat conductive material is not in contact with an acoustically active region of the second opposite face; (v) providing a heat sink material in contact to a second surface of the heat flow sensor, which second surface is not in direct contact with the heat conductive material; and, (vi) measuring the changes in mass of the sample and the flow of heat from the sample to the heat sink for the sample disposed on the first face of the piezoelectric substrate. In one embodiment, the microresonator has the capability to measure the mass of the sample applied to the first face, and the heat flow sensor has the capability to measure the flow of heat from the sample on the first face of the microresonator to the heat sink.
Yet another aspect of the present invention relates to methods for measuring the mass of a sample and the flow of heat from the sample to a heat sink, which methods comprise the steps of: (i) providing a quartz crystal microbalance capable of measuring the mass of the sample in contact with the quartz crystal microbalance; (ii) providing a heat flow sensor coupled thermally to the quartz crystal microbalance and capable of measuring the flow of heat from the sample to a heat sink; (iii) providing a heat sink coupled thermally to the heat flow sensor; and, (iv) measuring the changes in mass of the sample and the flow of heat from the sample to the heat sink for the sample disposed on the quartz crystal microbalance. In one embodiment of the methods, the quartz crystal microbalance comprises a quartz substrate having a perimeter, a first face for directly contacting the sample, and a second opposite face isolated from contacting the sample, the quartz substrate having a resonant frequency and capable of producing a measurement signal based on the resonant frequency. In one embodiment, the methods further comprise providing electrodes deposited on the first and second opposite faces of the quartz substrate, the electrodes being capable of supplying electrical signals to and from the quartz substrate. In one embodiment, the methods further comprise providing a heat conductive material extending in a continuous fashion from the perimeter of the quartz substrate to a first surface of the heat flow sensor, wherein the heat conductive material is not in contact with an acoustically active region of the second opposite face. In a preferred embodiment, the heat flow sensor comprises a thermopile. In one embodiment, the heat sink material is in contact to a second surface of the heat flow sensor, which second surface is not in direct contact with the heat conductive material. In a preferred embodiment, the quartz substrate is an AT-cut quartz crystal. In one embodiment, the heat conductive material provides a path for the application of radio-frequency power to the quartz substrate. In one embodiment, the heat conductive material comprises a metallic cylinder with a surface in contact to the first surface of the heat flow sensor. In one embodiment, the heat conductive material is brass.
As one skilled in the art will appreciate, features of one embodiment and aspect of the invention are applicable to other embodiments and aspects of the invention.